I did, I did! In fact, if you read further down my post that you quote, you'll see that I mentioned that triode AB1 can give up to 16w with enough B+. That's more or less what I'm doing now, except that my B+ is about 15v too low, so I get only about 14w even with fixed bias.
As to the question of which is probably better, EL34 class A pentode PP or EL34 class AB1 triode PP, IMHO the triode is the way to go. That's how I like it, using my speakers, listening to my favourite music. However, YMMV and, if you have the chance, you should try the different options and see which sounds best to you, with your speakers.
Other people's opinions are all well and good to avoid pitfalls but I don't think there are any serious pitfalls with the EL34. It's a very good pentode to begin with and never sounds bad as long as it's used sensibly, with good components (esp. the OPT, as alread stated).
ray_moth said:
Ok. one more question.
What about the EL84 in Class AB1 triode but using 4 valves per channel (PPP) instead of 2 EL34?
My reason: I am intending to use EL84 Class A Triode for the tweeter. With the option of going Class AB1 if I need more power. Using the EL84 instead of the EL34 would allow me to use the same tube.
Thanks.
I don't feel safe commenting on EL34s. The last time I used them was in a 10w PP UL amp that I built in 1961, based on the Leak Stereo 20. As you say, they should do the job on paper but how welll they're likely to behave in parallel PP I don't know. I'm sure someone else will be able to comment, though.
navin said:
You'll end up with somewhat less power (10 watts or so), but that's not audibly much different than the 15 watts you can get from EL34. The '84s have the advantage of low required plate voltage and high sensitivity- it won't take more than 9VRMS to drive the grids.
Again, spend a few minutes perusing the Mullard data sheets. They are astoundingly complete and comprehensive.
Hi Johan,
I can't remember where I found out where it said the 6CA7 was a tetrode, but it is not the same as an EL34. The plate structure is different. I know they have EL34's as wide as a 6CA7, but with labeling these days ... What I do know is that 6CA7's will handle plate dissipation better than EL34's do.
6L6GC's are sometimes shown schematically as a pentode too. I don't know what to make of it all. Yes the 6L6GC is a 30W plate. A 7581a has a 35W plate and sounds the same. I use those for Fender twins that like to destroy a 6L6GC. Surprisingly, 6L6EH's survive in Fender twins too. Go figure.
-Chris
I can't remember where I found out where it said the 6CA7 was a tetrode, but it is not the same as an EL34. The plate structure is different. I know they have EL34's as wide as a 6CA7, but with labeling these days ... What I do know is that 6CA7's will handle plate dissipation better than EL34's do.
6L6GC's are sometimes shown schematically as a pentode too. I don't know what to make of it all. Yes the 6L6GC is a 30W plate. A 7581a has a 35W plate and sounds the same. I use those for Fender twins that like to destroy a 6L6GC. Surprisingly, 6L6EH's survive in Fender twins too. Go figure.
-Chris
Hi SY,
Now you have got my curiousity up. The next HF-87 I get, I'll try this out. I want to see how it sounds, the amp doesn't really need to put out more power.
Thanks very much for the tips!
-Chris
Now you have got my curiousity up. The next HF-87 I get, I'll try this out. I want to see how it sounds, the amp doesn't really need to put out more power.
Thanks very much for the tips!
-Chris
One more thing... a way to keep the feedback more constant while reducing the excessive sensitivity is to knock down the open loop gain by subbing a 12AT7 for the 12AX7. The biasing wil need to be redone, but that presents another interesting opportunity- replacement of the 1k/25u cathode bias network of the input tube with a red LED. I haven't done this with my amp (just thought of it yesterday), but as soon as I'm back home, I'll try that out and let you know the results.
Hi there,
"replacement of the 1k/25u cathode bias network of the input tube with a red LED. "
I tried this with my ECC88 preamp. From what I remember the sound was a bit harsher and a bit grainy.
When researching LED biasing I read somewhere a review by someone who tried various biasing arrangements with a panel of HI-FI freaks, the simple resistor biasing was said to sound the best overall. Can't give you a link, but I destinctly remember their conclusion.
Hope that helps.
Shoog
"replacement of the 1k/25u cathode bias network of the input tube with a red LED. "
I tried this with my ECC88 preamp. From what I remember the sound was a bit harsher and a bit grainy.
When researching LED biasing I read somewhere a review by someone who tried various biasing arrangements with a panel of HI-FI freaks, the simple resistor biasing was said to sound the best overall. Can't give you a link, but I destinctly remember their conclusion.
Hope that helps.
Shoog
Hi SY,
Sounds interesting. Yes, I'm interested in what you find. Wish I had another HF-87 lying around. You have peaked my interest sir.
Shoog,
Be careful on what may be reported. While it may be true that it sounds harsher, it appears that since "SS is involved, sand sound comes out". Of course, it may have had this effect in your circuit too.
I haven't tried this, but have been able to refute many comments like this. Most times it comes down to people listening too hard or poor execution of the circuit. Please understand that there is no offense directed your way. I just like to keep an open mind with these things.
-Chris
Sounds interesting. Yes, I'm interested in what you find. Wish I had another HF-87 lying around. You have peaked my interest sir.
Shoog,
Be careful on what may be reported. While it may be true that it sounds harsher, it appears that since "SS is involved, sand sound comes out". Of course, it may have had this effect in your circuit too.
I haven't tried this, but have been able to refute many comments like this. Most times it comes down to people listening too hard or poor execution of the circuit. Please understand that there is no offense directed your way. I just like to keep an open mind with these things.
-Chris
Shoog said:
Hi Shoog,
a very interesting question. This was discussed at the European Triode Festival of the last year. Hartmut Quaschik noted in his report for TNT-Audio:
"Manfred Huber (Germany) elaborated on the various methods of providing bias voltage to a triode. In the following shootout the audience rated LOW-R bias best (that is using a low value resistor in the cathode and having additional current from an auxiliary supply - typically the dc heater supply - flowing to it, such that the desired bias would settle), then surprisingly Miss Piggy got a second price (capacitor bypassed cathode resistor), all the other methods (battery in cathode, battery in grid, LED bias, fixed voltage for grid and cathode to ground, non-bypassed cathode resistor) proved to have more or less uncomfortable sounding shortcomings."
Morgan Jones from Great Britain had made measurements on LED-Biasing. He was so kind to send me his favourite red diodes for that purpose (unfortunately I have forgotten the name of type) to Germany.
I tried the LED in some of my phonostages, but made it better later by coming back to the conventional circuit.
Best regards, Jo
anatech said:
Yes, Chris, it is all too much of a mess for comfort. My little effort to sort this out lead to aquiring dead 6L6s by Sovtek, EH, Svetlana, old 7581A etc. and dissect them. Main differences were the oval vs. rectangular anodes and different G1 cooling fins. But internal mounting pins were also thinner and longer in some Russian types etc. as I am sure you would have noticed. I believe the extent to which heat is conducted to the outside world (or not) via these makes a difference, thus my efforts on other threads to get NOS GE 6L6GCs. I am encouraged by your remarks regarding EH; my investigation also showed that they would probably last. The smooth box-type anode counted against rigidity under heat. All in all Svetlanas looked the best, but I have since read that folks lost a few. None is however as good (rigid) as 7581A or NOS 6L6GC by GE or RCA.
For the same reason I went off EL34s; with some the G2 will glow, with others not, in the same circuit. But this is somewhat off-thread; to sum up: We are stuck with present-day lack of consistency.
Johan
😡
FWIW, Bottlehead uses Agilent LEDs in their Seduction phono stage kit. The Agilent parts are quiet. What the exact drop is, I don't know.
An alternative to a LED for bias is a small NiMH rechargeable cell (1.2 V.). Charge fully before wiring such a cell in. The small trickle of current associated with operation keeps the cell charged up.
An alternative to a LED for bias is a small NiMH rechargeable cell (1.2 V.). Charge fully before wiring such a cell in. The small trickle of current associated with operation keeps the cell charged up.
Hi Eli,
Just like the old mercury cells from yesterday. I am still not comfortable with batteries determining bias in tube circuits. I imagine that any low noise source will be about the same if bypassed.
-Chris
Just like the old mercury cells from yesterday. I am still not comfortable with batteries determining bias in tube circuits. I imagine that any low noise source will be about the same if bypassed.
-Chris
Chris,
Those old Hg cells provided a STEADY 1.34 V., but would burst, IIRC, if a charging current was passed thru them. I would not use anything but NiMH or NiCd (which is toxic) in the cathode circuit. The current that's present while the unit is operating keeps the rechargeable charged.
In the case of "zero" current situations, such as tube grid and FET gate circuits, Li cell batteries seem to be best. FWIW, I'm quite partial towards Li battery bias in line level FET voltage follower circuits, as there's no head scratching about the bias voltage, and clipping at the gate is prevented. IMO, 9 or 12 V. from a Li battery beats the guano out of a Zener diode for forward biasing an enhancement mode MOSFET.
Those old Hg cells provided a STEADY 1.34 V., but would burst, IIRC, if a charging current was passed thru them. I would not use anything but NiMH or NiCd (which is toxic) in the cathode circuit. The current that's present while the unit is operating keeps the rechargeable charged.
In the case of "zero" current situations, such as tube grid and FET gate circuits, Li cell batteries seem to be best. FWIW, I'm quite partial towards Li battery bias in line level FET voltage follower circuits, as there's no head scratching about the bias voltage, and clipping at the gate is prevented. IMO, 9 or 12 V. from a Li battery beats the guano out of a Zener diode for forward biasing an enhancement mode MOSFET.
Hi Eli,
I was referring to grid circuits, didn't even consider cathode circuits. Scary thought. But you are right, the voltage was generally pretty steady. Li would be my choice for this too.
But batteries also may leak at higher temperatures. I will admit they do solve some engineering quandries. Years down the line the owner may even forget the battery is in there when something goes wrong.
-Chris
I was referring to grid circuits, didn't even consider cathode circuits. Scary thought. But you are right, the voltage was generally pretty steady. Li would be my choice for this too.
But batteries also may leak at higher temperatures. I will admit they do solve some engineering quandries. Years down the line the owner may even forget the battery is in there when something goes wrong.
-Chris
Shoog: I would think that it depends on how the LED is used and the current at which it's run. Admittedly, the low current of this stage doesn't put the LED onto the flattest part of its I/V curve. In experimenting, I'll start with this as the worst case, then increase the diode current with an adjustable CCS connected between B+ and the top of the diode. It should be interesting to observe the dependence of linearity (if significant) on current. My proposed simplification is getting more complicated than I'm comfortable with, but now I'm really curious...
Of course you woudn't use any device in it's non-linear region for this. You would feed the LED with enough current or use another device. Sometimes if there are no other choices, you are stuck with what you have.
-Chris
-Chris
"a very interesting question. This was discussed at the European Triode Festival of the last year. Hartmut Quaschik noted in his report for TNT-Audio:
"Manfred Huber (Germany) elaborated on the various methods of providing bias voltage to a triode. In the following shootout the audience rated LOW-R bias best (that is using a low value resistor in the cathode and having additional current from an auxiliary supply - typically the dc heater supply - flowing to it, such that the desired bias would settle), then surprisingly Miss Piggy got a second price (capacitor bypassed cathode resistor), all the other methods (battery in cathode, battery in grid, LED bias, fixed voltage for grid and cathode to ground, non-bypassed cathode resistor) proved to have more or less uncomfortable sounding shortcomings.""
Thanks for that, that was exactly the reference I was refering to.
SY I see your logic about supplying extra bias current to the LED. I would love to see this idea work, so I would be very interested in your eventual results. Still I am personally inclining to the KISS approach in all things, something about the elegancy which appeals to my simple nature. I also like the fact that simple resistors are self adjusting to a certain extent.
Shoog
"Manfred Huber (Germany) elaborated on the various methods of providing bias voltage to a triode. In the following shootout the audience rated LOW-R bias best (that is using a low value resistor in the cathode and having additional current from an auxiliary supply - typically the dc heater supply - flowing to it, such that the desired bias would settle), then surprisingly Miss Piggy got a second price (capacitor bypassed cathode resistor), all the other methods (battery in cathode, battery in grid, LED bias, fixed voltage for grid and cathode to ground, non-bypassed cathode resistor) proved to have more or less uncomfortable sounding shortcomings.""
Thanks for that, that was exactly the reference I was refering to.
SY I see your logic about supplying extra bias current to the LED. I would love to see this idea work, so I would be very interested in your eventual results. Still I am personally inclining to the KISS approach in all things, something about the elegancy which appeals to my simple nature. I also like the fact that simple resistors are self adjusting to a certain extent.
Shoog
ray_moth said:
what schematic do youuse for your EL34 Class A Triode. Can I switch this to Class AB1 (16W) easily so I cna have a low power and high power option? I think this might be the way to go for my tweeter amp.
For the woofer the a PPP EL34 Class A Triode would be adequate (with the option of using AB1 for more power).
ray_moth said:
given this would one recommend Class A Pentode over Class AB1 Triode (EL 34 Parallel Push Pull) for a woofer (the woofer is XOed at about 2500Hz).
my understanding.
a) some of the stridency of teh Pentode will not heard as the woofer is rolled off.
b) Using the EL34 in Class A (Triode or Pentode) permits less stable grid voltages
c) since the speaker in question is only 2 woofers connected in parallel (no crossover) damping factor of a Pentode mode might not be so critical.
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